Fixed an exception thrown from illegal characters in a regex

While writing a library to parse RFC 5322, the specification for a valid email address, I came across a bug that will cause an exception when creating regular expressions for character ranges.  Specifically it was from this (partial) case:

ctext = %d42-91

I am new to Clojure, so there are probably better and cleaner ways to solve this. Please feel free to modify and give critique, I welcome the feedback.